a. Field of the Invention
The present invention relates to a superprecision lathe, and more particularly to a superprecision lathe suitable for working, the surface of a thin plate-like object to be worked such as a magnetic disk.
b. Description of the Prior Art
In the case where the surface of a thin plate-like object to be worked, namely, a work such as a magnetic disk by using a superprecision lathe, a vacuum chuck for holding a such work by utilizing a vacuum suction power has been widely used. In the superprecision lathe of the vacuum chuck type, it is necessary to air-tightly connect the vacuum passage formed in the main spindle as a rotating member with the vacuum passage formed in a stationary member.
For example, the Japanese Unexamined Patent Publication No. 59-53137 discloses a superprecision lathe as shown in FIG. 1, in which a floppy disk used for storage medium in a computer is cut with a bite by rotating the floppy disk at a high speed. In the superprecision lathe, a vacuum chuck 13 for holding a work is mounted on one end of a rotating spindle 5 contactlessly supported in the air bearings 8 and 9 mounted on a frame. The spindle 5 has axially provided therein a through-hole 7 connected at one end thereof to the vacuum chuck 13. The other end of the spindle 5 is connected to a cylindrical joint 6 having a through-hole to be connected to the through-hole 7 of the spindle 5. The left half (as viewed on the illustration) of the cylindrical joint 6 is housed within a coupling casing 28 fixed on the frame by means of a supporting arm 29. The coupling casing 28 has provided in the left side wall thereof a hole 30 to be connected to a suction hose 31 of the vacuum pump and which is disposed oppositely to the end of the through-hole in the cylindrical joint 6. The coupling casing 28 has provided therein a permanent magnet 32 surrounding the cylindrical joint 6 and annular pole pieces 33 and 34 surrounding the cylindrical joint 6 with a small gap between the pole pieces and the joint. There is provided a magnetic fluid 35 between the end faces of the pole pieces 33, 34 and the cylindrical joint 6 to provide a sealing between the cylindrical joint 6 as rotating member and the coupling casing 28 as stationary member.
In the conventional apparatus so constructed that the evacuation passage of the spindle 5 is connected, as sealed by the magnetic fluid 35, to the evacuation passage of the stationary member, so it is necessary to recharge the magnetic fluid 25 from time to time when it runs short as scattered as the operation of the lathe goes on. The magnetic fluid is very expensive, and so the maintenance of such lathe is not economical. Furthermore, such scattering of the magnetic fluid 35 is likely to cause an undesirable influence on the surrounding members. Therefore, much care should be exercised during the maintenance and management of such lathe. Furthermore, in the horizontal type lathe, the supporting arm 29 is undesirably a vibration source depending upon the mechanical condition of the lathe.